Novel Space Alters Theta and Gamma Synchrony Across the Longitudinal Axis of the Hippocampus.

Hippocampal theta (6–10 Hz) and gamma (25–50 Hz and 65–100 Hz) local field potentials (LFPs) reflect the dynamic synchronization evoked by inputs impinging upon hippocampal neurons. Novel experience is known to engage hippocampal physiology and promote successful encoding. Does novelty synchronize or desynchronize theta and/or gamma frequency inputs across the septotemporal (long) axis of the hippocampus (HPC)? The present study tested the hypothesis that a novel spatial environment would alter theta power and coherence across the long axis. We compared theta and gamma LFP signals at individual (power) and millimeter distant electrode pairs (coherence) within the dentate gyrus (DG) and CA1 region while rats navigated a runway (1) in a familiar environment, (2) with a modified path in the same environment and (3) in a novel space. Locomotion in novel space was related to increases in theta and gamma power at most CA1 and DG sites. The increase in theta and gamma power was concurrent with an increase in theta and gamma coherence across the long axis of CA1; however, there was a significant decrease in theta coherence across the long axis of the DG. These findings illustrate significant shifts in the synchrony of entorhinal, CA3 and/or neuromodulatory afferents conveying novel spatial information to the dendritic fields of CA1 and DG targets across the long axis of the HPC. This shift suggests that the entire theta/gamma-related input to the CA1 network, and likely output, receives and conveys a more coherent message in response to novel sensory experience. Such may contribute to the successful encoding of novel sensory experience

An Information-Theoretic Approach to Deciphering the Hippocampal Code

Information theory is used to derive a simple formula for the amount of information conveyed by the firing rate of a neuron about any experimentally measured variable or combination of variables (e.g. running speed, head direction, location of the animal, etc.). The derivation treats the cell as a communication channel whose input is the measured variable and whose output is the cell's spike train. Applying the formula, we find systematic differences in the information content of hippocampal "place cells" in different experimental conditions. 1 INTRODUCTION Almost any neuron will respond to some manipulation or other by changing its firing rate, and this change in firing can convey information to downstream neurons. The aim of this article is to introduce a very simple formula for the average rate at which a cell conveys information in this way, and to show how the formula is helpful in the study of the firing properties of cells in the rat hippocampus. This is by no means the first a..